With help of hyperspectral imaging technology, a continuous spectrum can be divided into multiple spectral bands containing certain range of wavelengths, therefore, desired spectral bands can be further analyzed individually more in detail. Hence, hyperspectral imaging technology is widely applied to technical fields such as remote sensing, food science and bio-medical research. A previous hyperspectral imaging system is based on laboratory setups, therefore it slows in analysis speed, high in cost and not compact at all. In order to solve the above problems, a hyperspectral imager was implemented by monolithically integrating a wedge filter on top of a CMOS image sensor by the researchers of Interuniversity Microelectronics Center (IMEC).
As presented by IMEC, the method for fabricating a hyperspectral image sensor is by monolithically integrating a wedge filter on top of the photosensitive region of a CMOS image sensor. Such kind of hyperspectral image sensors, compared to the laboratory style ones, have much smaller volume, faster analysis speed and lower cost. A filter integrated on the top of the CMOS image sensor is generally a set of Fabry-Pérot interferometers (Fabry-Pérot interferometers), as shown in FIG. 1. The Fabry-Péinterferometers include two highly reflective layers (mirrors), a bottom reflecting layer 11 and a top reflecting layer 12; and a transparent cavity layer is placed in between the two reflecting layers 11 and 12. The transparent cavity layer 13 is a wedge structure, and each step height corresponds to one spectral band.
The transparent cavity layer defines the cavity length where light can be reflected in between the two mirrors.
In conventional technology as IMEC implemented, the transparent cavity layer with step structures is fabricated by alternating photolithography-etching steps. For example, a transparent cavity layer with eight step heights is fabricated with three sets of photolithography-etching processes, as shown in FIG. 2.
A height of the cavity layer determines a wavelength of a central spectrum of a corresponding filter, hence in a multi-step Fabry-Pérot interferometer (for example, one with more than 100 step structures), the heights of the cavity layer must be fabricated very precisely. Practically, it is reported that etching depths have within wafer non-uniformity of at least about 2.7% during each etching process, which means that the more etching steps are performed, the greater the non-uniformity. As a transparent cavity layer composed of multiple step structures requires multiple etching steps, thus, accumulation of the non-uniformity increases as the etching steps increases. Therefore, the device may be failed due to being out of the specifications.
In addition, it is difficult to apply plasma etching or wet etching to etch some specific materials, for example, hafnium(IV) oxide (HfO2). Since the boiling points of the halides of the lanthanide transition metals (including Hf) are high (higher than 300 degrees Celsius), it is difficult to etch those halides up through plasma etching. A reagent based on containing rich hydrogen fluoride (HF) molecules may be used for wet etching of HfO2, however the reagent containing HF normally will destroy the interface layer of photoresist and HfO2. Therefore, it may result of lifting off the photoresist. To overcome the issue, a hard mask has to be added, thereby it increases process steps, cost and uncertainties. Thus, if the transparent cavity layer is fabricated by photolithography-etching processes, the selection of materials for the transparent cavity layer is restricted due to etching process.